In:
ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2020-02, No. 61 ( 2020-11-23), p. 3073-3073
Abstract:
We will discuss several examples in which photoexcitation of a stable radical anion or cation with visible or NIR light results in production of a high-potential radical ion excited state that can carry out difficult redox reactions relevant to artificial photosynthesis. First, we will show that selective excitation of a naphthalenediimide radical anion (NDI •− ) covalently linked to the 4-, 5, or 6-positions of the bipyridine (bpy) in the Re(bpy)(CO) 3 X (X = Cl or pyridine) carbon dioxide reduction catalyst results in electron transfer from 2* NDI •− to Re(bpy)(CO) 3 X to form Re(bpy •− )(CO) 3 X, the first intermediate in the photocatalytic reduction of CO 2 . Femtosecond UV/Vis, near-IR and mid-IR spectroscopy on these constitutional isomers show that systematically varying the electronic coupling as well as the reaction free energy increases the lifetime of Re(bpy •− )(CO) 3 X by an order of magnitude when the NDI chromophore is attached to the 6-position of bpy. Electrochemical reduction of the corresponding Mn(bpy)(CO) 3 X CO 2 reduction catalyst is thought to proceed by the initial reduction of Mn I to Mn 0 . We have covalently attached a naphthalenediimide radical anion (NDI •- ) chromophore to the 4-, 5-, or 6-positions of the bpy via a phenyl bridge to produce Mn(NDI •- -bpy)(CO) 3 X, where X = Br, CH 3 CN, or DMF, and have used femtosecond and nanosecond transient IR spectroscopy to directly observe the intermediates produced by two electron transfer reactions following selective photo-excitation of NDI •− . In complexes where NDI •− is attached at the 4- or 5-positions of bipyridine, only the reaction Mn( 2* NDI •− -bpy)(CO) 3 X → Mn(NDI-bpy •− )(CO) 3 X is observed, while in the complex where NDI •− is attached to the 6-position of bipyridine, the reaction sequence: Mn( 2* NDI •− -bpy)(CO) 3 X → Mn(NDI-bpy •− )(CO) 3 X → Mn 0 (NDI-bpy)(CO) 3 is observed. Moreover, in the complexes with an NDI •- bound to the 6-position of bipyridine, Mn 0 (NDI-bpy)(CO) 3 exhibits a lifetime that is ~10 5 times longer than those in complexes with an NDI •- bound at the 4- or 5- positions of the bipyridine. On the oxidative side, we will discuss the 10-phenyl-10H-phenothiazine radical cation (PTZ +• ), which has a manifold of excited doublet states accessible using visible and NIR light that can serve as super-photo-oxidants with excited state potentials is excess of +2.1 V vs SCE to power energy demanding oxidation reactions. Photo-excitation of PTZ +• in CH 3 CN with a 517 nm laser pulse populates a D n electronically excited doublet state that decays first to the unrelaxed lowest electronic excited state, D 1 ' ( τ 〈 0.3 ps), followed by relaxation to D 1 ( τ = 10.9 ± 0.4 ps), which finally decays to D 0 ( τ = 32.3 ± 0.8 ps). D 1 ' can also be populated directly using a lower energy 900 nm laser pulse, which results in a longer D 1 ' → D 1 relaxation time ( τ = 19 ± 2 ps). To probe the oxidative power of PTZ +• photoexcited doublet states, PTZ +• was covalently linked to each of three hole acceptors, perylene (Per), 9,10-diphenyl-anthracene (DPA), and 10-phenyl-9-anthracene-carbonitrile (ACN), which have oxidation potentials of 1.04, 1.27, and 1.6 V vs. SCE, respectively. In all cases, photoexcitation of PTZ +• result in ultrafast oxidation of Per, DPA, and ACN. The photoexcited peri -xanthenoxanthene radical cation (PXX +• ) is another super-oxidant that has a 124 ps electronic excited state (D 1 ) lifetime and can deliver +2.1 V vs. SCE of oxidizing potential. Photoexcitation of PXX +• covalently attached to electron deficient 9,10-bis(trifluoromethyl)anthracene (TMFA) using an 885 nm laser pulse drives oxidation of TFMA with unity quantum yield. Extending the PXX +• -TFMA dyad to a molecular triad having a 9,10-diphenylanthracene terminal hole acceptor, PXX +• -TFMA-DPA, and selectively exciting PXX +• results in formation of PXX-TFMA-DPA +• with a 46% quantum yield and a τ = 11.5 ± 0.6 ns lifetime. This work demonstrates that the PXX +• D 1 electronic excited state can serve as a promising super-oxidant for challenging oxidation reactions relevant to solar-energy applications.
Type of Medium:
Online Resource
ISSN:
2151-2043
DOI:
10.1149/MA2020-02613073mtgabs
Language:
Unknown
Publisher:
The Electrochemical Society
Publication Date:
2020
detail.hit.zdb_id:
2438749-6
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